This invention relates generally to a neutral point clamped (NPC) or diode clamped multilevel converter, and, more specifically, to a method of operation of a multilevel converter.
A diode clamped multilevel converter is generally used in high power industrial applications such as variable speed drive (VSD) systems or in energy conversion applications such as a solar (or photovoltaic) power generating systems or wind turbine generators. Power losses of diode clamped multilevel converters are an important issue in unit sizing of drive systems and photovoltaic systems because of the influence of such losses on the total deliverable energy. Power losses occur in multilevel converters mainly because of losses in switching devices such as Insulated Gate Bipolar Transistors (IGBTs), Gate Turn Off (GTO) Thyristors, and Integrated Gate Commuted Thyristors (IGCTs), which are generally used in such converters. Power losses also occur in the converters due to the presence of clamping diodes and passive components such as filter inductors.
Clamping diodes may be used in multilevel converters to block or clamp the voltage across the switching devices to a certain level. For reasons of modularity and simplification, IGBT modules are frequently used in place of neutral-point clamped diodes to perform the blocking function. In such embodiments, the IGBTs are constantly gated off. Whether diode modules or IGBT modules are used, each of them has some internal inductance, which further increases power losses in a converter during commutation of switching devices.
The switching devices generally have three major types of losses: conduction losses, switching losses, and gate drive losses. The switching losses correspond to the losses that occur during state changes of the switching device (during turn on and turn off). The conduction losses correspond to losses that occur in the switching device during its conduction (when the device is carrying a current). Gate drive losses refer to the energy required to charge and discharge gate-source and gate-drain capacitances of the switching devices and are affected by switching frequency, gate capacitance, and the voltage traversed. However, usually switching losses and conduction losses are the major factors in high power IGBT applications.
Therefore, it is desirable to provide a method and a system that will address the foregoing issues.